I spent two hours last night reading and learning about optics and telescope mirrors. Took some good notes in my new leather journal Erin gave me a few months ago. It’s got a short leather strap that acts as a tie-up to close the thing. Very Henry(Indiana)-Jones looking.

I’ve always been confused by the relationship between focal length, mirror size, and f/ratio on telescope mirrors. Often designers and scope nuts go off on tangents on the benefits of an f/5 scope vs an f/8 or f/11 scope. I’ve never internalized what they mean by this so I quickly get lost when they start talking about it. I understand f/ratios when it comes to photography, but mirrors are a little different. So I decided last night to figure it out once and for all. And I did!

In a nutshell, the smaller the f/number (called the “f/ratio”) is, the shorter the telescope tube will be. What makes a smaller f/number? A more curved mirror. Think of it this way: if you have a flat mirror the light that hits it and bounces off will never come to a focus (f/infinity). Grind a little bit out of the center of the mirror glass, making it more curved, and the light rays will reflect off of it and meet somewhere far away from the mirror (lets say 100′). As you start to grind more and more material out of the mirror, making it more curved, the light rays will start to be focused to a common point that is closer to the mirror. Eventually, the mirror will be so curved that the reflected rays will focus, oh, about four feet away (just an example).

So let’s say you decided to stop grinding your 8″ mirror once the light rays focus four feet (48″) away. Congratulations, you have an f/6 mirror. Why? Because 48″ (the focal LENGH) divided by 8″ (the diameter of the mirror), is 6… so they would call your mirror an f/6.

It’s easy to grind a mirror without much curvature to it, but this will mean a really long telescope. For example, an 8″ mirror that focuses light 120″ away would be an f/15 mirror… and the scope would have to be 10 feet long! Remember, you have to actually look at the light at the point it all focusses together, which means some sort of secondary mirror or eyepiece at that focal point, which means a long telescope tube to support all that hardware. Much better to spend more time making the mirror more curved so you don’t have a behemoth of a telescope.

So why not keep grinding away until you have a bowl-shaped mirror that can focus, say, 12″ away? In this case, a 12″ focal length mirror that is itself 8″ in diameter would be an f/1.5 mirror. But you’ve taken off so much glass from the middle of the mirror that it is quite literally bowl-shaped. These extremely curved mirrors are not only hard to make accurately, but the can be weak in the center (flexing!) and they suffer from something called “spherical aberration”. Basically, the image won’t look very good because the light rays are bent so dramatically.

Most telescope makers/mirror grinders recommend a happy medium of around f/5 or f/6. So if you’ve decided on an 8″ mirror, you get a final telescope length of around 40-48 inches. I think I’m going to go with a 6″ mirror in mine because the raw materials are a little cheaper. An f/5 or f/6 mirror means the final tube length will be 30″ to 36″ long, or about three feet (5×6 or 6×6). I think this will strike a nice balance between size, ability, portability, and cost. A 6″ mirror ground to f/5 would cost around $150-200 if I purchase it outright. If I do it myself I’d pay about 1/3 that price. Of course, grinding a mirror can take 50 hours, so I’m not really “saving” that much. I’m still not sure if I’m going to grind it myself or buy. I’m not really to that decision point just yet.

But I have decided on a six inch reflecting telescope along the lines of this basic design:

Light goes in the front, reflects off the curved mirror, and converges at the secondary mirror where it is reflected to the eyepiece. It’s called a “Newtonian reflector” and is a good balance between capability and size. It’s also not too hard to build. Most Newtonians have round tubes that you can construct from- no kidding – cardboard cement tubes you buy at the hardware store. I may get a cardboard tube to test the optics and focal length, but the final design will have a wooden tube, probably octagonal or multi-segmented. Inlay? Decoration? Reinforcement to avoid wood movement? Time will tell.

July 25, 2007

Amateurs in the sciences are probably no less numerous than in the arts. Music may have more devotees than botany, but surely there are more amateur astronomers than Sunday painters. One is impelled to ask: what are these amateurs all seeking? It is this: an expansion of their understandings and of their capacities, and the pleasure that derives from effort. One can appreciate the arts without ever having touched a brush or a musician’s bow; similarly, one may keep abreast of progress in science merely by reading. But the purely receptive role is not the one that yields the richest fruit. If that which we acquire is to penetrate deeply, we must in some degree be participants: we must use our eyes to observe, we must experiment, we must build with our own hands. The extensive knowledge contained in books must sound within us echoes of personal experience. Only in this way can a truly cultivated understanding be developed.

July 22, 2007

My friend, vocal/actor extraordinaire, and all around good guy Adam Creighton came over today to finish tweaking his character demo in the studio. This has been an invaluable exercise in learning the idiosyncrasies of the new setup as well as getting some hands-on time with the 5.0 version of Digital Performer.

We spent about 4 hours auditioning and adding sound effects and music to his sixty second character demo. I’m gearing up to offer demo recording packages to local voiceover talent as part of the studio remodel. Adam’s will serve as an example of what kind of service I can provide. I’m rather pleased with the results. Take a listen.

Rolf is a friend of mine from years ago. He and I spent a couple summers guiding wilderness trips in the Rockies. He then turned his peripatetic leanings into a travel/writing career.

Vagabonding is a pitch-perfect travelogue/guide about how to travel the world with minimal funds and minimal stress. Rolf focuses on the “why’s” of long-term travel as well as the “how’s”. Lots of good information, funny anecdotes, and inspiring reasons to hit the road.

Started a new book, The Last Colony, just after dinner tonight at 7:30. Little did I know it was one of those books. It’s 2:10 and I just finished it. Normally this wouldn’t be a problem but insomnia and a lost earplug conspired to only grant me 4 hours of sleep last night. Even though I was positively droopy a few times, I couldn’t put it down book was that good.

Totally worth it. It’s the third book of John Scalzi’s trilogy of Old Man’s War, The Ghost Brigades, and The Last Colony. Highly recommended.

July 18, 2007

“I’ve have one record left that I owe a major label, then I will never be seen in a situation like this again. If I could do what I want right now, I would put out my next album, you could download it from my site at as high a bit-rate as you want, pay $4 through PayPal. Come see the show and buy a T-shirt if you like it. I would put out a nicely packaged merchandise piece, if you want to own a physical thing. And it would come out the day that it’s done in the studio, not this “Let’s wait three months”

Economist Juliet Schor estimated that for every hour of TV a person watches each week, he or she will increase his or her annual spending by about $200, according to a 1999 article in the Spokane, Wash., Spokesman-Review.

Yahoo has an interesting article titled “How to Earn $1 Million by Not Watching TV” wherein they posit that if you gave up TV, and all associated costs, and invested the money, you would make a million over the course of your lifetime. While I applaud the notion, I have to say that their estimate of $700/month for TV costs seems a bit excessive. Still, the article makes me feel all superior and all, so there ya go.

I only wish it were true! But giving up TV has resulted in something much more valuable than mere money. We’ve reclaimed time for ourselves (please insert obligatory disclaimer stating that we don’t hold it above anyone that we don’t watch TV. If you get enjoyment out of it, knock yourself out. Not making a value judgement for others here).

Last night Erin and I spent a delightful 4 hours or so not talking to each other. No, we weren’t fighting (that’s exceedingly rare anyway). We were each seduced by our own books. As part of my telescope research I plowed through the book “The Neptune Files” about astronomers William Herschel, John Couch Adams, and Urbain-Jean-Joseph Le Verrier and the discovery of Uranus and Neptune. I never knew there was such a fracas over who got primacy for the discovery. Herschel discovered Uranus the old fashioned way: by looking through a telescope, but Adams and Le Verrier both independently deduced the position of the next planet out, Neptune, by calculating the invisible hand of gravity on Uranus. This took over 10,000 pages of extremely dense calculations and will probably stand as one of the greatest feats of calculation by a single person (Matt, correct me here if I’m wrong). What’s more amazing is that both astronomers did the calculations independently with very little support (Adams’ brother did watch over the astronomer’s shoulder to make sure he didn’t make any basic math mistakes). In the end, both astronomers came up with very nearly the same answer, pointed their scopes at the sky, and found the distant orb. Well, there’s more to it than that, but you’ll just have to read the book.

Erin, on the other hand, is trying to plow her way through the Harry Potter books again before H-Day on saturday. We’re getting a special release day delivery from Amazon and we probably will have a rare fight over who gets to read book 7 first. 🙂 I’m amazed at her pace through the rest of the books. She started at book 2 about eight days ago and is now in book 5. Last night, in a single sitting, she read 200 pages to finish book 4, got up, went upstairs, and came down with Order of the Phoenix twenty seconds later.

My point is that, two years ago, we’d have wiled the time away staring blankly at the tube (please see disclaimer above if you’re feeling like I’m getting all self-righteous on you). I much prefer this way of living!

But anyway, yeah, the article. I wonder if they calculated the added cost of buying books into their figures? I think that’ll more than offset the savings.